The bodies of 12 people, including a woman, were found in clandestine graves in Chihuahua, a state in northern Mexico, prosecutors said.

Investigators found the bodies of six men and a woman Monday in Guachochi, a city in the southern part of the state, which borders the United States, the Chihuahua Attorney General's Office said.

The victims appear to have been tortured, the AG's office said.

One of the victims was shot in the face and the rest were strangled to death, AG's office spokesman Carlos Gonzalez told Efe, citing the results of autopsies on the bodies.

"The victims, all of whom had been missing since Aug. 13, have been identified by relatives and their ages ranged from 19 to 40," Gonzalez said.

The bodies were buried one on top of the other in a shallow grave, the AG's office spokesman said.

Five human skulls, meanwhile, were found in a clandestine grave at a site in the Juarez Valley about 60 kilometers (37 miles) from Ciudad Juarez.

Investigators are searching for the rest of the bodies, the coroner's office said.

Chihuahua, one of Mexico's most violent states, has registered a total of 2,600 homicides since January, state Attorney General Carlos Manuel Salas told state legislators last week.

Of the total number of homicides, 222, or 8.5 percent, involved women, the state AG said.

The northern state is home to Ciudad Juarez, a border city located across the Rio Grande from El Paso, Texas, that is considered Mexico's murder capital.

Chihuahua has accounted for nearly 30 percent of the more than 40,000 deaths registered in Mexico since President Felipe Calderon declared war on the country's drug cartels shortly after taking office in December 2006.

Mass graves have been found in different parts of Mexico this year.

Over the weekend, investigators in the central state of Mexico, which surrounds the Federal District and forms part of the Mexico City metropolitan area, found five bodies in a clandestine grave in the city of San Francisco Tlalcilalpan.

The bodies were found in San Francisco Tlalcilalpan's Santa Cruz de los Patos neighborhood, a Mexico state Attorney General's Office spokesman told Efe.

The bodies were in an advanced state of decomposition, the AG's office spokesman said.

Investigators found 218 bodies in mass graves in Durango city, the capital of the like-named northern state, in April and May.

The first mass grave was found in a residential area on April 4 and yielded 89 bodies.

Investigators suspect that the killings may have been carried out by the Sinaloa, Los Zetas and Beltran Leyva drug cartels, media reports said.

The majority of the bodies discovered in Durango city were in the Las Fuentes neighborhood.

The Sinaloa cartel, Mexico's oldest and largest drug trafficking organization, has been trying to gain control of Durango, the press reported.

Durango, one of the states most affected by drug-related violence, is said to be the hiding place of Sinaloa cartel boss Joaquin "El Chapo" (Shorty) Guzman.

Nearly 200 bodies were found earlier this year in clandestine graves in Tamaulipas, a state in northeastern Mexico that has been rocked by drug-related violence.

A total of 47 clandestine graves containing 193 bodies were found in April and May.

The mass graves were found following reports that gunmen had forced men off buses headed for Reynosa, a city across the border from McAllen, Texas, between March 19 and March 31.

The bus passengers were grabbed in a bid to "identify possible members" of the Gulf cartel, which has been battling the rival Los Zetas cartel for control of smuggling routes into the United States, some of the suspects arrested in connection with the killings told investigators.